1. Field of the Invention
This invention relates to an acceleration sensor chip having improved sensitivity.
2. Description of the Related Art
Techniques are under development for manufacturing minute mobile structures of approximately several hundred μm using micromachining technology such as semiconductor microprocessing techniques. For example, these techniques are applied to various sensors, optical communication switches, radiofrequency (RF) components, and so on.
The mobile structure can be manufactured using a conventional semiconductor manufacturing process, and can be integrated onto a single chip.
A chip on which a system having a specific function, including the above described mobile structure, is constructed is known as a Micro Electrical Mechanical System (MEMS) or a Micro-System-Technology device (MIST devices). These systems and devices are referred to simply as MEMS devices hereafter.
An acceleration sensor (see Japanese Patent Application Kokai (Laid-Open) No. 2004-198243, for example) is known as the MEMS device.
Referring to FIGS. 9A to 9C of the accompanying drawings, an outline of the constitution of the conventional acceleration sensor disclosed in Japanese Patent Application Kokai No. 2004-198243 will be described.
FIG. 9A is a schematic plan view illustrating the upper face of constitutional elements of a conventional acceleration sensor chip package. FIG. 9B is a schematic cross-sectional view of the acceleration sensor chip package, taken along the dot-dash line 9B—9B in FIG. 9A. FIG. 9C is a schematic diagram showing another cross-sectional view of the acceleration sensor chip package, taken along the dot-dash line 9C—9C in FIG. 9A.
A piezo-type acceleration sensor 110 disclosed in Japanese Patent Application Kokai No. 2004-198243 includes a peripheral portion 112 which supports a weight fixing portion 114a flexibly, and four weight portions 114b which are fixed to the weight fixing portion 114a. The weight fixing portion 114a is connected to the peripheral portion 112 by four beam portions 116. The beam portions 116 are provided with piezo elements 136 for detecting displacement of the weight portion 114b. 
The outer shape of the upper surface of the piezo-type acceleration sensor 110 and the shape of the upper surface of an opening portion 150 receiving the weight fixing portion 114a and weight portions 114b are square.
A triangular stopper 119 is provided above a certain part of each weight portion 114b to restrict displacement of that weight portion 114b. 
The groove 150, peripheral portion 112 and weight portions 114b are square, and the entire acceleration sensor chip is square when seen from its upper surface side.
An acceleration sensor chip is sometimes packaged with other chips such as a control chip for controlling operations of the acceleration sensor chip. In recent years, there is a demand for reductions in the size and thickness of the devices in which acceleration sensor chip packages are provided. Thus, there is a demand for further reductions in the size and thickness of the acceleration sensor chip packages.
When attempts are made to achieve further reductions in the size and thickness of the acceleration sensor chip, the volume of the weight portion, i.e., the mass of the weight portion, must inevitably be reduced. This results in a decrease in the detection sensitivity with respect to acceleration, vibration, tilting, and so on.